GENEVA (AFP) – A United Nations expert on Friday urged the Sri Lankan government to set up an independent probe into the authenticity of a video clip aired in Britain allegedly showing Sri Lankan troops executing prisoners.

Philip Alston, the UN Special Rapporteur on extra-judicial, summary or arbitrary executions, said Colombo had categorically denied the allegations, but stressed the need for an investigation.

"If the government?s position is validated as a result of an inquiry, the international community can rest easy and the government will have been vindicated," he said.

"There is no justification for not moving ahead with such an investigation in view of the government?s confidence that such atrocities were never perpetrated by its armed forces," he added.

The images, which he described as "horrendous," indicate a serious violation of international law if found to be authentic, he said.

Alston also pointed out that he had asked permission to visit Sri Lanka on several occasions in recent years, but Colombo had not given him the green light.

The video footage, aired Tuesday by Channel 4 in Britain, was allegedly shot during the final stages of the army's defeat of Tamil Tiger separatists.

The rebels were finally vanquished in May after nearly four decades of ethnic bloodshed.

Sri Lanka's military had said the video was a fabrication designed to "discredit" its armed forces.

Widespread international concern was voiced over the number of civilians killed during the last leg of the fighting, while aid groups now fear for the welfare of 300,000 Tamils held in the state-run camps.

Philip Alston, the UN Special Rapporteur on extrajudicial, summary or arbitrary executions, said he was aware that the Sri Lankan Government had categorically denied the allegations raised by the video, which has been aired this week.

"If the government`s position is validated as a result of an inquiry, the international community can rest easy and the government will have been vindicated," he said.

"There is no justification for not moving ahead with such an investigation in view of the government`s confidence that such atrocities were never perpetrated by its armed forces," he added.

“These images are horrendous and, if authentic, would indicate a serious violation of international law,” Professor Alston said in a statement, noting that the Government’s denial “makes it all the more important for an independent investigation to be set up.

“If the Government’s position is validated as a result of an inquiry, the international community can rest easy and the Government will have been vindicated. There is no justification for not moving ahead with such an investigation in view of the Government’s confidence that such atrocities were never perpetrated by its armed forces.”

Earlier this year Government forces declared victory over LTTE after years of fighting in the small South Asian country.

Professor Alston added that he regretted that the Government had not yet issued him an invitation to make an official visit to Sri Lanka, despite a number of requests in recent years, but he hoped an invitation may come given the new allegations.

Like many other UN rapporteurs, Professor Alston reports to the UN Human Rights Council in Geneva and to the General Assembly and he serves in an independent and unpaid capacity.